The present invention relates to the field of the processing of digital data for applications such as the compression of digital data, the search for digital data, the comparison of digital data or the decompression of digital data. The invention relates to the processing of audiovisual data and more generally any type of digital data. The aim of the invention is to reduce the processing time and the computing resource requirements with regard to both the computing power and the memory requirements.
The applications more particularly but not exclusively relate to the processing of images requiring a very large number of data to describe them. To reduce the transmission time and the size required for storage, information is compressed by extracting the visual information that alone will be coded. This coded information must be located in an optimum way in terms of frequency and in space to enable an optimum restitution while avoiding any redundancy detrimental to the performance of the coding. For this purpose, using the wavelet transform technique is known, the co-ordinates of which constitute a vector network that is then subjected to a vector quantization step.
The principle of vector quantization (VQ) is to code a sequence of samples forming a vector instead of coding each of the samples individually. The coding is done by approximating the sequence to be coded by a vector belonging to a catalogue of forms usually referred to as a “codebook”. Each of the vectors of the codebook is indexed. During coding, the index of the nearest vector to the sequence of samples to be coded will be used to represent it.
The known solutions require the determination of each of the vectors, the recording thereof in a memory and then the execution of processing on all the vectors in order to count them. The vector base may require several gigabytes and the computing times for such a large base are lengthy. The aim of the invention is to propose a counting and indexing method avoiding these drawbacks.
The Application for the International Patent WO9933185 is known in the prior art, relating to a coding method consisting in determining a vector, referred to as the leader vector, which comprises the same components as the quantized vector but in a predetermined order, and then in determining the level or rank of said quantized vector in said formed set of vectors that have the same components as the leader vector and are, in said set, ordered in a predetermined manner. The method then consists in forming a code from on the one hand, an index representing said thus determined leader vector and on the other hand said rank. The main difficulty encountered for designing an algebraic vectorial quantiser for the compression is related to the problem of counting and indexing vectors in the regular point network (which constitutes the quantization dictionary). We present here the solutions that we have proposed for solving these problems in the case of a generalised Gaussian distribution source (such as for example the wavelet coefficients).